The present invention relates to a non-inverting amplifier circuit and, in particular, to a non-inverting amplifier circuit having a low input impedance.
With comparable external circuitry, a differential amplifier connected as an inverting amplifier and a differential amplifier connected as a non-inverting amplifier exhibit different amplification factors and different input impedances. In some applications, it is desirable to have an amplifier circuit with the characteristics of an inverting amplifier in which the input impedance Ri approaches zero, and in which the input terminal can be used as a current summing junction, but which circuit does not invert the input signal. In such case, it is customary to connect two inverting amplifiers in series.
It has been found that a non-inverting amplifier circuit obtained by connecting two inverting amplifiers in series is critical with respect to the frequency and phase behavior of the circuit, a characteristic which may be particularly detrimental if the circuit is used in a closed control loop. Such an arrangement may tend to self-oscillate, thus requiring additional stabilization measures. This tendency toward oscillation occurs because the lowpass filters formed by the circuit and transistor capacitances of two series-connected differential amplifiers produce a steep phase curve resulting in a critical phase shift in the input signal at lower frequencies than would be the case with only one differential amplifier. Moreover, the gain of the series-connected differential amplifiers is the product of the no-load gains of the individual amplifiers. Consequently, the greater phase shift and the higher no-load gain produces conditions in which there is a high probability that self-oscillation or resonance will occur in a closed control loop.
If, for example, such an amplifier is disposed in the feedback path of a further amplifier in order to compensate for the high attenuation produced by an adjusting member, the tendency to oscillate is substantially reduced in a single-stage amplifier. This occurs because the phase shift required for oscillation can occur only at substantially higher frequencies. Reduction of the initially low no-load gain associated with increasing frequencies prevents the loop amplification required for oscillation from being reached.
Even if the arrangement is not operated in a closed loop, the long signal delay and the lack of phase constancy over a given frequency range can be a disadvantage. It is therefore an object of the present invention to provide a non-inverting amplifier which has an input impedance and a frequency dependent phase shift between the output signal and the input signal which is almost equal to or less than that of a comparable inverting amplifier.